WO1999016098A1

WO1999016098A1 - Semiconductor photoelectric surface

Info

Publication number: WO1999016098A1
Application number: PCT/JP1998/004119
Authority: WO
Inventors: Tokuaki Nihashi
Original assignee: Hamamatsu Photonics K.K.
Priority date: 1997-09-24
Filing date: 1998-09-11
Publication date: 1999-04-01
Also published as: EP1024513B1; DE69807103D1; AU9002998A; EP1024513A4; JPH1196896A; DE69807103T2; EP1024513A1

Abstract

A phototube (10) comprises a photocathode (30) having photoelectric surface. In a sealed enclosure (20) whose inside is vacuum, the photoelectric cathode (30) and an anode (40) are opposed to each other. Voltages are applied to them through lead pins (51 and 52). The photocathode (30) includes a metallic support plate (31) to which is secured a sapphire plate (32) on which are formed an a-AlN matching layer (33), a p-type GaN active layer (34), and a CsO surface layer (35). The active layer (34) has a dopant concentration that increases from 1 x 1016 cm-3 in the surface up to 5 x 1017 cm-3 at a depth of 100 nm. The dopant concentration only at the deepest region over a thickness of several nanometers is 1 x 1018 cm-3. The crystallinity of the active layer (34) is improved, and the diffusion length is increased, improving the quantum efficiency and sharp-cut property.

Description

Akira Itoda Semiconductor Photocathode Technology

The present invention relates to a semiconductor photocathode that emits photoelectrons into a vacuum upon incidence of photons, in particular a III-V semiconductor photocathode. Background art

It is preferable that a semiconductor photocathode used for a photomultiplier tube has a high photoelectron emission efficiency. One such semiconductor photocathode is disclosed in US Pat. No. 3,387,161. This semiconductor photocathode has an active layer obtained by activating the surface of a p-type semiconductor having a doping concentration of 1 × 10 ¹⁸ cm ³ or more and 1 × 10 ¹⁹ cm ″ ³ or less with an alkali metal. With this configuration, downward energy band bending is formed on the vacuum emission side surface of the photocathode, which lowers the vacuum level barrier on the surface to facilitate the escape of photoelectrons and separates from the vacuum emission side surface. Even photoelectrons generated inside the active layer are more likely to reach the emission side surface because the diffusion length can be increased without lowering the electron emission probability.

However, when the doping concentration is increased, absorption near the band edge occurs due to defects or the like generated in the crystal, and the sharp cut property of the light absorption characteristics tends to be impaired. Further, from the viewpoint of solar blindness, it is preferable that the doping concentration is low. This is because the lower the doping concentration, the more the decrease in crystallinity can be suppressed. However, when the dopant concentration is low, the diffusion length can be increased, but the probability of electron emission decreases, resulting in a decrease in quantum efficiency. For this reason, conventionally, It was difficult to further reduce the concentration.

In view of the above problems, an object of the present invention is to provide a semiconductor photocathode having a low doping concentration and a high quantum efficiency.

The semiconductor photocathode of the present invention is a semiconductor photocathode which emits photoelectrons into a vacuum in response to incident photons, wherein the surface on the photoelectron emission side is a P-type doped III activated with alkali metal or alkali metal oxide. An active layer comprising a Group V compound semiconductor is provided, and the surface doping concentration on the photoelectron emission side of the active layer is 1 × 10 ¹⁷ cm ³ or less.

According to this, since a decrease in crystallinity is prevented, the diffusion length increases. In addition, since the crystal is good, the probability of electrons reaching the emission side surface is high, the deterioration of the electron emission probability can be prevented, and the quantum efficiency can be kept high.

Further, the energy band gap of the active layer is preferably at least twice the work function of the alkali metal or alkali metal oxide of the surface layer. In this case, electrons are easily emitted from the surface.

An electron supply layer may be provided on a side different from the photoelectron emission side of the active layer.

Alternatively, the doping concentration of the active layer may be 1 × 10 ¹⁷ cm ³ or less in the vicinity of the photoelectron emission surface and 1 × 10 to 1 × 10 cm ″ ′ on the back side.

Alternatively, the doping concentration of the active layer may be gradually increased from the vicinity of the photoelectron emitting surface toward the back, and the doping concentration at the deepest portion on the back side may be 1 × 10 to 1 × 10 cm.

According to these configurations, the diffusion length is further increased, and the electric field inside the diffusion layer is configured to move the electrons toward the emission surface side, so that the probability of the electrons reaching the emission surface is improved.

More preferably, the thickness of the region having a doping concentration of 1 × 10 ¹⁸ to 1 × 10 ¹⁹ cm ′ ³ on the back side of the active layer is several nm or less. In this case, of the photoelectrons generated in the high-concentration doping layer, the amount of photoelectrons that migrate to the side opposite to the emission-side surface and disappear is suppressed. For this reason, Applicable to over-type photocathode structure.

Alternatively, a Schottky electrode formed on the surface of the active layer may be provided, and an external bias may be applied to the active layer. According to this, the photoelectrons generated inside the active layer due to the external bias are efficiently guided to the emission side surface.

The present invention will become more fully understood from the detailed description and the accompanying drawings, which _follow are presented by way of illustration only and should not be taken as limiting the invention.

Further areas of applicability of the present invention will become apparent from the detailed description below. However, while the detailed description and specific examples illustrate preferred embodiments of the present invention, they are provided by way of example only, and various modifications within the spirit and scope of the present invention may be made. It is obvious that variations and modifications will be apparent to those skilled in the art from this detailed description. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic diagram of a phototube using the photocathode of the present invention.

FIG. 2 is a diagram showing a dopant concentration distribution of the active layer on the photocathode of FIG.

FIG. 3 is a diagram comparing wavelength characteristics of the photoelectric surface of the present invention and a conventional product.

FIG. 4 is a graph showing the relationship between the dopant concentration and the quantum efficiency.

FIG. 5 is a diagram showing an example of the dopant concentration distribution of the active layer of the photocathode of the present invention. FIG. 6 is a diagram showing another example of the dopant concentration distribution of the active layer of the photocathode of the present invention.

FIG. 7 is a diagram showing still another example of the dopant concentration distribution of the active layer of the photocathode of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram of a transmission type phototube using a semiconductor photocathode according to the present invention. The phototube 10 is configured such that a photocathode 30 and an anode 40 using a semiconductor photocathode according to the present invention are accommodated in a sealed container 20 whose inside is evacuated. This vacuum container

2 0 is a hollow cylindrical glass container, inside thereof is kept below a pressure of about 10 ^'8 Torr. The photocathode 30 is supported by metal lead pins 51 via a metal support plate 31 having a hole at the center and a metal support base 50. On the other hand, the anode 40 is a metal electrode formed in a rectangular frame shape, and is supported by a metal lead pin 52. The lead pins 51 and 52 penetrate the bottom of the vacuum vessel 20 and are connected to external power sources, respectively, and apply a voltage higher than that of the photocathode 30 to the anode 40.

The photocathode 30 is a substrate formed of sapphire on a rectangular frame-shaped metal support plate 31.

32 is fixed, and a matching layer 33, an active layer 34, and a surface layer 35 are sequentially laminated thereon.

The matching layer 33 is made of, for example, amorphous A 1 N grown on the substrate 32 by epitaxial growth. The matching layer 33 has a thickness of about 10 nm, and is lattice-matched with the active layer 34 to allow the active layer 34 to grow well. Further, it is provided for the purpose of preventing backward movement of photoelectrons generated in the active layer 34.

The active layer 34 is formed from p-type GaN epitaxially grown on the matching layer 33. The thickness of the active layer 34 is 100 nm or more, and Mg or Zn is doped as a p-type dopant. Its concentration distribution is as shown in Fig. 2.It has a first layer with a thickness of lOOnm near the surface and a second layer with a thickness of lnm formed at least deeper than the light incident surface. In the first layer, the dopant concentration near the surface is 1 × 10 ¹⁶ cm ³ , the concentration increases toward the second layer, and the concentration at the boundary with the second layer is 5 × 10 ¹⁷ cm ³ Reach The dopant concentration in the second layer is 1 × 10 ¹⁸ cm ³ higher than in the first layer.

The growth of the matching layer 33 and the active layer 34 is performed by MOCVD, MBE, HWE, etc. Various crystal growth methods can be used.

On the surface of the active layer 34, a surface layer 35 made of an alkali metal or an oxide thereof, for example, Cs or Cs0 is formed by vapor deposition. This surface layer 35 is formed as a monoatomic layer. The energy band gap of the vacuum discharge of the surface layer 35 when C s is used as the metal is 1.4 eV, and when C s O is used, the energy band gap is 0.9 eV. The band gap is less than half of 3.4 eV.

Next, the operation of the photoelectric tube will be described. When light enters from the substrate 32 side of the photocathode 30, the incident light passes through the hole of the metal support plate 31, passes through the substrate 32, the matching layer 33, and enters the active layer 34. Reach. Photons are absorbed mainly in the first layer of the active layer 34 to generate photoelectrons. The distribution of the band gap energy in the active layer 34 substantially corresponds to the dopant concentration. As a result, the photoelectrons generated in the first layer move in the first layer so as to slide down the slope and reach the surface layer 35, and have a large band gap with the surface layer 35. Is extremely thin, so it is easily released into a vacuum. The emitted photoelectrons reach the anode 40 by an electric field between the photocathode 30 and the anode 40 and are detected as a current.

The present inventor compared the wavelength characteristics of the conventional photocathode and the photocathode of the present invention shown in FIG. The results are shown in comparison with FIG. Here, a comparison was made with a conventional product in which the active layer was one layer and the dopant concentration was 1 × 10 ¹⁸ cm ³ . The broken line shows the wavelength characteristics of the quantum efficiency of the photocathode of the present invention, and the straight line shows the wavelength characteristics of the quantum efficiency. The product of the present invention has a higher quantum efficiency at a wavelength of 350 nm or less than the conventional product, has a low quantum efficiency at a wavelength of 400 ηιη or more, improves sharp cut properties, and improves characteristics in a low wavelength region. It was confirmed that. This is thought to be due to the fact that, as the diffusion length increases, the probability of photoelectrons reaching the surface increases due to the improvement in crystallinity, thereby improving the photoelectron emission efficiency from the surface.

Figure 4 shows various prototypes with different dopant concentrations in the active layer. 5 is a graph comparing quantum efficiency at 254 nm. The quantum efficiency varies considerably depending on the prototype, but the overall high dopant concentration (1 ιιο ¹⁸ to 1X)

It was confirmed that the quantum efficiency of the product having a low dopant concentration (1 × 10 ¹⁷ cm ³ or less) of the present invention was equal to or higher than that of 10 ¹⁹ cm ′ ³ ).

Next, another embodiment of the present invention will be described. The concentration distribution of the active layer 34 may be composed of a first layer and a second layer as shown in FIG. 5 in addition to that shown in FIG. 2, and the concentration of each may be changed stepwise. . With this configuration, photoelectrons generated by photons incident from the opposite side of the emission surface can be effectively guided to the emission side.

Further, the photocathode of the present invention can be applied to a reflection photocathode which emits photoelectrons on the same side as the incident direction of photons. In this case, the matching layer 33 may be formed of, for example, amorphous A 1 N or GaN epitaxially grown on the substrate 32. FIGS. 6 and 7 show the concentration distribution of the active layer on the reflection type photoelectric surface corresponding to the transmission type photoelectric surface of FIGS. 2 and 5, respectively. In either case, photoelectrons generated in the high dopant concentration layer can be efficiently guided to the emission side surface.

Control of these dopant concentrations can be easily set by controlling the supply of the dopant material. Although it is preferable to provide a high-concentration region in a portion away from the emission surface, it is not essential and may not be provided. Alternatively, by applying an external bias voltage to the active layer, the internal energy-bandgap level may be graded to force photoelectrons to the emission surface. In this case, the internal dopant concentration may be uniform, or the predetermined distribution may be provided as described above.

In the above description, an example using G a N as the active layer has been described. However, G a, In, A l, B, etc. are used as group III materials, and N, P, As, etc. are used as V group materials. Can be used.

In addition, as the alkali metal of the surface layer, Cs, Cs0, etc. can be used. You.

As described above, according to the present invention, the active layer having a low dopant concentration stabilizes the crystallinity and increases the diffusion length, so that the photocathode having high quantum efficiency and improved sharp cut property can be obtained. can get.

Furthermore, by using a semiconductor having a wide energy band for the active layer, photoelectrons are reliably emitted from the surface.

In addition, by adjusting the dopant concentration distribution of the active layer, photoelectrons generated in the back of the active layer can be reliably guided to the emission side surface.

It is apparent from the above description of the invention that the present invention can be variously modified. Such modifications cannot be deemed to depart from the spirit and scope of the invention, and modifications obvious to those skilled in the art are intended to be within the scope of the following claims. Industrial applicability

The photoelectric surface according to the present invention can be applied not only to a photoelectric tube but also to a photoelectric surface performing various photoelectric conversions.

Claims

The scope of the claims

1. A p-type doped III-V compound in which the surface on the photoelectron emission side is activated by an alkali metal or an alkali metal oxide on a semiconductor photocathode that emits photoelectrons into a vacuum in response to incident photons. A semiconductor photocathode having an active layer made of a semiconductor, wherein a doping concentration of at least a surface of the active layer on the photoelectron emission side is 1 × 10 ¹⁷ cm ³ or less.

2. The semiconductor photocathode according to claim 1, wherein the energy band gap of the active layer is at least twice the work function of the metal or the metal oxide of the surface layer.

3. The semiconductor photocathode according to claim 1, further comprising an electron supply layer on a side different from the photoelectron emission side of the active layer.

4. The doping concentration of the active layer is 1 × 10 ¹⁷ cm ³ or less near the photoelectron emission surface and 1 × 10 ¹⁸ to l × 10 ¹⁹ cm ³ on the back side. The semiconductor photocathode as described.

5. The semiconductor photocathode according to claim 4, wherein the depth of the region having a doping concentration of 1 × 10 ¹⁸ to 1 × 10 ^lacm ³ on the back side of the active layer is several nm or less.

6. The concentration of the dopant in the active layer is gradually increased from the vicinity of the photoelectron emission surface toward the back, and the concentration of the deepest part on the back side is 1 × 10 ¹⁸ to 1 × 10 ¹⁹ cm ³ . 2. The semiconductor photocathode according to claim 1, wherein:

7. The semiconductor photocathode according to claim 6, wherein a thickness of a region having a doping concentration of 1 × 10 ¹⁸ to 1 × 10 ¹⁹ cm ³ on the back side of the active layer is several nm or less.

8. The semiconductor photocathode according to claim 1, further comprising a Schottky electrode formed on a surface of the active layer, wherein an external bias is applied to the active layer.